This invention relates to computer disc drive actuator assemblies, and more particularly to the bearings used to support an actuator arm housing in such an assembly.
The computer industry traditionally records information on disc drives. The disc drive normally includes one or more discs which are rotated, with information magnetically recorded on the surface of the disc along tracks. Information is read by a magnetic transducer or head which is supported on a flex arm or gimble assembly of an actuator arm. Pivoting of the actuator arm is used to move the transducer radially between tracks along the disc surface. It is essential that the positioning of the transducer at a particular track be accomplished both accurately and reproducibly, to ensure that the proper information is read by the transducer.
There has been an ever increasing desire to minimize the space required by computer components and to maximize the speed with which computers can perform their various functions. To meet these desires, disc drive designers have sought to decrease the distance between tracks on a disc, and to decrease the time required to accurately and reproducibly position the transducer. Accordingly, the accuracy of the actuator arm is required to be greater and greater.
Actuator arms are generally supported by a system of ball bearings around a shaft, such that the actuator arm rotates about the shaft. Typically two axially-spaced conventional roller bearings are used to support the housing of the actuator arm about the shaft. The first step in the typical assembly of the actuator arm housing onto the shaft is to attach the inner race of a roller bearing against a shoulder on the shaft. Second, the housing is attached around the roller bearing such that the outer race butts up against a shoulder on the housing. The axial relationship between the shaft and the actuator arm housing is thus set through attachment of this first roller bearing. Thirdly, a second bearing unit is placed around the shaft and between the shaft and the housing.
Commercially available roller bearings often have a small amount of looseness or "free play" both in the axial and radial direction. That is, the inner race can be displaced with respect to the outer race merely by changing the direction of axial and radial force transmitted by the bearing. However, bearing free play can be extremely detrimental to the performance of the disc drive. In particular, radial free play of the pivot assembly can allow the transducer to be moved to the wrong location on the disc, such that information is distorted, read improperly or, worse yet, the wrong information is read.
In additional to the radial accuracy of the pivot assembly, the radial stiffness of the pivot assembly must be taken into account. The pivot assembly must have proper stiffness such that the starting, moving and stopping of the transducer can be done quickly and without significant vibration of the transducer. Similar to the free play problem, vibration can cause distortion or improper reading of information even if the transducer is in the proper location. The pivot assembly/actuator arm must be stiff enough so that any residual vibration is small in magnitude, and must further not create any critical frequencies or resonances within the band width of the servo system.
Various methods have been used to try to improve the radial performance of the bearing mechanism in actuator arm pivot assemblies. Perhaps the simplest method is to require a higher degree of accuracy in the design and construction of the bearing mechanism, such that the resulting bearing mechanism has a smaller amount of free play. However, increasing the bearing mechanism precision to a sufficient level can be quite costly, and other methods have been attempted to improve the performance of the bearing.
Another method to improve the radial performance of a bearing mechanism is to "take up" (i.e., reduce or eliminate) the free play by applying a "preload" force in the axial or radial direction. The preload force is intended to prevent a change in the direction of the force transmitted by the bearing, such that the inner race maintains a constant displacement from the outer race. Accordingly, the preload force should normally be greater than the forces transmitted by the bearing during use.
In the case of spherical ball bearings, much of the free play is caused by the smaller diameter of the balls moving within the larger diameter curvature of the raceways. Applying an axial preload will cause the balls to centralize themselves in a particular portion of the raceways of the bearings, and eliminate both axial and radial free play. The preload will also cause some compression of the balls and raceways affecting the stiffness of the pivot assembly. If two bearing units are used, generally one bearing unit will be preloaded in one direction and the second bearing unit preloaded in the other direction, such that the preload forces transmitted by the bearings offset each other with no resultant force on the pivot assembly. With a proper axial preload present, the pivot assembly can be used without the balls of either bearing leaving the particular portion of the raceways, and axial and radial free play is eliminated. However, to eliminate free play while still obtaining the desired stiffness and endurance characteristics of the bearings, it is essential that the preload force be accurately set and maintained.
In configuring a pivot assembly for axial preloading of one bearing against another, placement of three of the races is not critical. The first three races may be attached by any method against the outside of the shaft or the inside of the housing. It is the force placed on the fourth and final bearing race which determines the preloaded of the bearings against each other. Accordingly, preloading is generally accomplished by placing a defined axial force on the fourth race after the other three races have been axially positioned.
One known method for placing an accurate preload force on the bearings is to allow the fourth race to be loose-fit, either against the shaft or the housing. The placement of the first three races is set such that these races can withstand a significant amount of axial force without moving. A preload spring is then placed against the fourth race to provide the desired axial preload. The spring places a known and controlled axially directed force on the fourth race. Because the fourth race is loose-fit, the shaft or housing places no force on the fourth race, and the entire preload is carried through the bearing mechanism. When the preload is transmitted through the bearing and no other forces are present, an equal and opposite axial preload must carried between the housing and the shaft through the second bearing element. Thus both bearing elements are preloaded with the identical force applied by the spring. The spring must be adequately placed so that it will continue to provide the preload force against the fourth race throughout the life of the pivot assembly.
However, the loose-fit of the fourth race causes its own problems in the radial accuracy of the bearing mechanism. The loose-fit fourth race is essentially in a bi-stable state (i.e., it is either leaning in one direction or the other), and any force which causes the fourth race to wobble between states will cause radial inaccuracy in the actuator arm and transducer.
The problem of the loose-fit fourth race has been addressed by applying lock-tight adhesive between the fourth race and the shaft or housing to which the fourth race is attached. The assembly of a pivot assembly which uses adhesive is largely the same as the loose-fit assembly described above. However, after the preload spring force is applied to the fourth race, the engagement between the fourth race and the shaft is cemented by adhesive. Because the set adhesive will carry the proper preload force, the spring for a glued fourth race may be removed after the adhesive sets up. Alternatively, a dead weight may be used to place the proper preload on the fourth race as the adhesive sets, and the dead weight may likewise be subsequently removed.
Using a glued fourth race eliminates the bi-stable tendency of the loose-fit fourth race described above, but also creates its own problems. Pivot assemblies are generally assembled in clean rooms, and the introduction of the adhesive to the clean room tends to be messy and difficult. It is difficult to control the application of the adhesive, both to ensure that adhesive fully extends on the necessary surfaces, as well as to ensure that no additional adhesive is applied which could seep out to contaminate the clean room conditions. Adhesive application problems become particularly egregious if the adhesive should enter the bearing structure and prohibit the bearing from working properly. Additionally, the long term stability of the adhesive is not always acceptable. If the adhesive deteriorates, the fourth race might again become loose, destroying the accuracy of the pivot assembly.
Accordingly, it is desired to find a method of placing the proper preload onto the bearings of the pivot assembly which will not lead to the problems discussed. It has generally been believed that the fourth race cannot be press-fit or otherwise rigidly attached, as there was no way to ensure placement of the proper axial preload force onto such a rigid attachment.